For passive radiative cooling applications, all-day harvested cold energy is an essential performance indicator concerning its long-term and continuous operation. To this end, high mid-infrared emissivity of emitter surface is a key parameter for steadily boosting the net cooling power regardless of the time-varying environment. In this work, hexagonal-array grating-patterned PDMS film on a silver reflector is proposed for efficient 'all-day' radiative cooling. The optimized grating structure achieves the weighted emissivity of 0.985 in the atmospheric transparent window with the solar reflectivity of 0.933, leading to the predicted all-day average cooling power of 85 ~ 130 W/m(2). We also show from outdoor experiments that the proposed structure can produce 4.1 6.3 W/m(2) extra all-day cooling power compared to a flat PDMS emitter. Furthermore, the grating-patterned surface introduces self-cleaning feature that is desirable for long-term usage.